Urethane acrylate is widely used in coating, inks, pressure sensitive adhesives, adhesives or the like as an ultraviolet ray (UV) curable resin composition. Among these, by having a polybutadiene or polycarbonate skeleton, adhesion, water resistance, chemical resistance, flexibility, elasticity and toughness are imparted, and in recent years, extensive studies have been performed even in the fields of optical members, electronic materials and semiconductors. Typically, such a urethane acrylate is synthesized by reacting a polybutadiene polyol or a polycarbonate polyol with a polyisocyanate to obtain a polyurethane having hydroxyl groups or isocyanate groups at both terminals and by further reacting with hydroxyl group-containing acrylate or isocyanate group-containing acrylate (Patent Documents 1 to 7). In addition, a synthetic method in which hydroxyl group-containing acrylate and a polyisocyanate are reacted, and then, the product is attached to the terminals of a polyol having a polybutadiene skeleton or a polycarbonate skeleton is also proposed (Patent Documents 8 and 9).
However, in these urethane acrylates having a polybutadiene skeleton or a polycarbonate skeleton, the glass transition temperature (Tg) is lower than room temperature in many cases, and when applying the resin composition to a substrate and curing by irradiation with active energy rays such as ultraviolet rays, there were disadvantages that stickiness remains on the surface of the coating film, and it is difficult to become tack-free. To solve the residual stickiness, using in combination with a monofunctional or polyfunctional acrylic monomer or a photosensitizer has been reported. For example, Patent Document 1 proposes a photocurable type resin composition obtained by blending 20% to 80% of a monofunctional acrylate and 2% of a sensitizer, and in Patent Document 2, a modified acrylate-based urethane prepolymer is synthesized from trimethylol propane, and this is mixed with a monofunctional acrylate to be used. Since urethane acrylate having a polybutadiene skeleton or a polycarbonate skeleton has strong hydrophobicity, there were problems in which solubility in many acrylic monomers is insufficient, and flexibility by use of polyfunctional acrylate deteriorates. Furthermore, since the acrylate group of the urethane acrylate resin has an ester structure, hydrolysis is likely to occur, moisture resistance, water resistance, durability, and the like are poor, and the curing speed when irradiated with active energy rays is also low.
To improve active energy ray curability of urethane acrylate, a urethane acrylamide oligomer has been proposed (Patent Document 11). By changing the polymerizable group from an acrylate group to an acrylamide group, ultraviolet ray curability was improved and the stickiness of a cured film surface was improved, but there was no mention of the solubility in a general-purpose organic solvent and an acrylic monomer or the transparency of the obtained cured film.
In Patent Document 8, one serious disadvantage of urethane acrylate having a polybutadiene skeleton was pointed out. This is that cloudiness is likely to occur before and after UV curing. In a case where cloudiness occured, urethane acrylate was not applicable to an optical member requiring transparency, and light transmittance was low, and thus, UV did not reach up to the inside of a resin, and curing was insufficient in some cases. As an improvement plan of the patent document, an urethanization reaction by a tin-based catalyst was mentioned, but a risk of causing deterioration of long-term storage stability, corrosion in the case of being used in an electrical equipment or a short circuit, by the remaining catalyst was newly generated.
On the other hand, in Patent Document 5, a pressure sensitive adhesive for optical members is proposed, and in Patent Document 6, urethane acrylate having a carbonate skeleton as a UV-curable type resin composition used in semiconductor elements or liquid crystal display devices was proposed. These patent documents paid attention to the heat resistance and the moisture resistance of the polycarbonate skeleton, and provided a composition that can cope with a high temperature and high humidity environment, but there was no mention of transparency essential to an optical member or the like, staining properties to the substrate of a protective film having a pressure sensitive adhesive, or the like.
Patent Document 7 proposed a method of irradiating in two stages in which a laminate of a photocurable resin formed of urethane acrylate having a polybutadiene skeleton and (meth)acryl-based monomer is produced, then, the laminate is irradiated with black light at an irradiation amount of 10 to 300 mJ/cm2, and further irradiated with active energy rays at an irradiation amount of 500 to 5,000 mJ/cm2. Patent Document 10 adopts a UV curing method of the same two stage irradiation with respect to urethane acrylate having a polycarbonate skeleton. This method had objects to reduce the residual monomer after photocuring by slowly curing, to improve curability, to prevent curing shrinkage, and to make distortion between the coating film and the substrate difficult to occur. However, since this method causes complication of step by two stage irradiation and increase in cost of the product, this method is not preferable.